There has been a device configured to emit white light and made up of a combination of a semiconductor light emitting element, such as a blue LED, and a phosphor configured to emit luminescence by light emitted from the LED, and heavy use of this device as an illumination light source is recently made. Such white light emitting devices can be roughly classified into those using a blue LED as an excitation source and those using a near ultraviolet LED as an excitation source.
In a white light emitting device using a blue LED as an excitation source, a blue LED and a yellow-emitting phosphor are combined, or a blue LED, a green-emitting phosphor and a red-emitting phosphor are combined, so that white light is generated. However, part of emission of a blue LED is used as a component of white light emitted from the device. Therefore, the device is readily affected by the unevenness in characteristics of the blue LED, and the emission spectrum can be controlled in the limited wavelength range by the phosphor. These make it difficult to control color rendering properties in the case of using the device as an illumination light source. Accordingly, there has been room for improvement. On the other hand, in a white light emitting device using a near ultraviolet LED having a wavelength around 400 nm as an excitation source, white light can be made up of a combination of phosphors of three kinds, that is, blue-emitting, green-emitting and red-emitting. Therefore, this device has advantages in that control of the emission spectrum is relatively easy, and that a light source having high color rendering properties is readily obtained in the case of using the device for illumination applications.
In the case of using a near ultraviolet LED as an excitation source, a phosphor activated by divalent europium (Eu2+), such as BaMgAl10O17:Eu or (Ba,Sr,Ca,Mg)10(PO4)6Cl2:Eu, is known as an example that can be used as a blue-emitting phosphor. As an example of a green-emitting phosphor, a phosphor activated by divalent europium (Eu2+), such as (Sr,Ba)2SiO4:Eu, SrGa2S4:Eu, Ba3Si6O12N2:Eu or Sr3Si13Al3O2N2:Eu is known. Further, as a red-emitting phosphor, a phosphor activated by Eu2+, such as CaAlSiN3:Eu or (Sr, Ca)2Si5N8:Eu, and a phosphor activated by trivalent europium (Eu3+), such as La2O2S:Eu. Among these phosphors, many red-emitting phosphors activated by Eu2+ give high light emitting efficiencies even when excited by near ultraviolet LEDs. However, the red-emitting phosphors exhibit strong absorption in the visible light region. Therefore, there is room for improvement in that light applied from the white light emitting device is colored reddish, and that it is difficult to design the amount of coating when a desired spectrum is to be obtained.
On the other hand, a red-emitting phosphor activated by Eu3+ and a red-emitting phosphor activated by Eu2+ exhibit completely different emission spectra as illustrated in FIG. 1. The emission spectrum of a red-emitting phosphor activated by Eu3+ has characteristics that most of emission energy is concentrated within the wavelength range from 600 to 630 nm. Therefore, regarding this red-emitting phosphor, the emission components are reduced in a wavelength range of low efficacy that is positioned on the side of a wavelength longer than this wavelength range. Consequently, this red-emitting phosphor can achieve an emission spectrum that is advantageous in terms of sectral luminous efficacy even if the total sum of its emission energy is equal to that of the red-emitting phosphor activated by Eu2+. From such reasons, red-emitting phosphors activated by Eu3+ such as Y2O3:Eu and Y2O2S:Eu have conventionally been used as major red-emitting phosphors for use in fluorescent lamps and color cathode-ray tubes. Further, a Eu3+-activated red-emitting phosphor has small light absorption in the visible light region, and therefore its spectrum design is relatively easy, which can reduce coloring of a white light emitting device that is remarkable in the case of using a red-emitting phosphor activated by Eu2+. However, compared to a red-emitting phosphor activated by Eu2+, absorption of the Eu3+-activated red-emitting phosphor with respect to near ultraviolet excitation light is generally weak, which leads to an inferior light emitting efficiency of a white light emitting device. This has been problematic.
Several kinds, not one kind, of structure of a white light emitting device using a plurality of phosphors are known. For example, the simplest structure uses a phosphor layer in which a plurality of phosphors are dispersed in a mixed form in a resin within the same layer. To attain a higher light emitting efficiency, there is proposed a white light emitting device using a three-layer structure phosphor layer in which the longer the wavelength of a phosphor, the closer to the excitation LED the phosphor is, whereas the shorter the wavelength of a phosphor, the farther from the excitation LED the phosphor is. Further, there is proposed a white light emitting device using a two-layer structure phosphor layer in which a first layer containing a red-emitting phosphor and a second layer containing a green-emitting phosphor and a blue-emitting phosphor laminated on an excitation source. However, because of reabsorption by phosphors, and the like, any conventionally known white light emitting device has not yet achieved a sufficient light emitting efficiency. It has been desired to attain a higher light emitting efficiency.